1. Field of the Invention
The present invention relates to a magnetic head arrangement and a method of detecting a magnetic property of an object by using the same. More particularly it relates to a magnetic head arrangement used for non-destructive test of properties such as construction, stress and fatigue from the magnetic property of the object and a method of detecting the magnetic property of the object by using the same.
2. Description of the Related Art
It has been attempted to non-destructively test a material of an object by using a nature that a magnetic property of the material depends on a crystal grain size, a construction of precipitates and stress. Namely, when an object is magnetically excited, the magnetization of the object changes and the construction of the object and the state of stress can be detected from the change in the magnetization. For example, a permeability is measured to estimate a tensile strength of a steel or a corecivity is measured to estimate a quenching strength. Recently, a method which uses a Barkhausen noise due to the discontinuity of magnetization has been attracting a notice and a method of estimating a fatigue of mild steel by using such a method (L. P. Karjalainen et al, IEEE Tans. Mag. MAG16, 514(1980)) and a method of estimating a stiffness of tool steel (Nakai et al, Iron and Steel, Vol. 75, No. 833 (1989)) have been proposed.
As methods of magnetically exciting an object, there are known a current passing method, an electric conduction method, a coil method and a yoke method. Among them, the yoke method offers an advantage that it allows relatively simple magnetic excitation by merely applying it to a portion of a relatively large product such as iron or steel products manufactured on line or existing building, and it has been for the test of material. On the other hand, as a method of detecting a change in the magnetization of the object, there are known a coil method, a yoke method, a Hall element method and a reluctance element method. The coil method and the yoke method are convenient since they are less affected by a temperature of the object.
FIG. 8 shows a prior art magnetic head used to detect a magnetic signal and a signal processing system therefor. The magnetic head comprises an excitation head core 51, a detection head core 52, and excitation and detection coils 53 and 54 wound on the respective cores. A material of the excitation head core 51 and of the detection head core 52 is soft magnetic material such as silicon steel, permaloy or soft ferrite.
In order to detect magnetic signal, a ramp wave or an AC sinusoidal wave is supplied from a voltage wave-form generator 58 to the excitation coil through an amplifier 57 and the magnetic head core 51 is brought in contact with the surface of the object 55 to locally magnetize it. A magnetic flux 56 is generated in the magnetized object in the direction of magnetization. When the detection head core 52 having a higher permeability than that of the object is in contact will the surface of the object, a part of the magnetic flux flows into the detection head core and a voltage is induced in the detection coil 54.
A biggest shortcoming of the prior art magnetic head is that when the magnetic head is moved off the object, the strength of the detected magnetic signal abruptly attenuates. This is because a gap is formed between the detection head and the object when the detection head is moved off the object and a magnetic reluctance increases and the magnetic flux 56 no longer flows into the detection core. Accordingly, when the prior art magnetic head is used, the magnetic signal having a high S/N ratio cannot be produced if a distance between the detection head and the object is larger than approximately several tens .mu.m.
Usually, an oxide layer of several tens .mu.m to several hundreds .mu.m is present on the surface of the object. Further, in the on-line measurement, the magnetic head should be spaced from the surface of the object by several hundreds .mu.m for movement in measurement. Accordingly, it is hard to apply the prior art magnetic head to such a circumstance.
JP-A-60-57247 discloses the use of soft ferrite for the magnetic excitation core and the detection head core and an I-shape core for the detection head core. In this patent application, a radius of curvature is imparted to a portion of the core which is to be in contact with the object in order to reduce a measurement error due to a misarrangement thereof when an operator applies the magnetic head to the surface of the object. Accordingly, it is based on the contact measurement and it makes the non-contact measurement difficult because of the attenuation of the signal intensity.
The inventors of the present invention have proposed in JP-A-6-194,342 corresponding to Japanese Patent Application No. 4-357663 a compound magnetic head in which a tip end of the detection head facing the object is formed by a shape variable magnetic material. By using such a magnetic head, the magnetic signal can be detected even if the magnetic head is spaced by approximately 0.2 mm. However, in order to apply it to the on-line test, it is necessary to further increase the allowable distance between the magnetic head and the object.
A total voltage waveform induced by the detection head includes a voltage waveform having the same frequency as that of the excitation superposed with a small high frequency voltage waveform. The total voltage waveform induced in the detection head is herein called a total induced voltage waveform. A permeability and a corecivity may be determined from the total induced voltage waveform. The superimposed small high frequency voltage waveform is called a Barkhausen signal and it is derived from the discontinuous change in the magnetization. Since these magnetic properties have a correlation with the construction of the object, the load stress and the fatigue, they are important parameters in diagnosing the material of the object.
Since the magnitude of the Barkhausen signal waveform is very small relative to the magnitude of the total induced voltage waveform, it is usually impossible to amplify only the Barkhausen signal waveform to a sufficient level within an analysis dynamic range of an oscilloscope 61. Thus, in order to extract only the Barkhausen signal waveform, in the prior art system, a low frequency component of the total induced voltage signal is removed by a frequency filtering device 59 and the resulting signal is amplified by a voltage amplifier 60. Thus, when the prior art magnetic head is used, several additional devices are required to detect the Barkhausen signal, which leads to the increase of the weight and power consumption of the system. Further, as the number of elements increases, more spurious noise is introduced, which makes the measurement of a fine signal more difficult.
For example, Mishima et al detect the waveform of the Barkhausen noise by using a magnetic head having a soft ferrite core. However, it is required to use in the detection system a frequency filtering device because the positional relationship between the excitation head and the detection head is not yet optimized (Piping Technology Vol. 35, No. 2, 1993, pp. 86-89).
Further, C. G. Gardner et al detect the Barkhausen noise by using a magnetic head using an air-core detection coil (International Journal of Non-destructive Testing, 1971, Vol. 3, pp. 131-169). In this case too, the voltage waveform induced in the air-core coil is applied to filtering processing through a frequency filtering device in order to extract the Barkhausen waveform.